Stabilized multivibrator oscillator



June 24, 1952 M. E., MOHR 2,601,444

STABILIZED MULTIVIBRATOR OSCILLATOR Filed Oct. 12, 1949 lNl/ENTOR M. E.MOHR ATT R/VEV Patented June 24, 1952 STABILIZED MULTIVIBRATOROSCILLATOR Milton E. Mohr, New Providence, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application October 12, 1949, Serial No. 121,031

9 Claims.

This invention relates to oscillatory circuits and is especiallyconcerned with improved methods and means for stabilizing relaxationtype oscillatory circuits against unwanted frequency variations.

It is well known that the characteristics of many types of oscillatorsand particularly those of the relaxation type are subject to changesarising from variations in the circuit potentials, and also from changesin the relative positions of the electrodes of the included electrondischarge devices. It is, of course, desirable that the oscillationfrequency of these circuits be made independent of such circuit changesin so far as is possible. It is accordingly an object of this inventionto make possible improved multivibrator type of oscillatory circuits inwhich frequency instability is reduced to a minimum.

It is also an object-of this invention to make possible stabilizedrelaxation type oscillators, the operation and frequency output of whichare substantially unaffected by variations in the anode potentialsupply.

It is a further object of this invention tomake possible multivibratortype of oscillatory circuits in which the effect of variations in thecut-off and saturation potentials caused by aging of the electrondischarge device is effectively nullified.

In accordance with the invention, in one of its preferred embodiments, apair of electron discharge devices, which may take the form of pentodeor beam-power tetrode vacuum tubes, are arranged with the usualresistance-capacitance cross coupling between the anode of each tube andthe control-grid electrode of the other tube to form a conventionalmultivibrator oscillator. The biasing potential for each controlgridelectrode is made variable with changes in the magnitude of the anodepotential source. Also, it is so proportioned with respect to the anodepotential supply that the voltage changes that are injected into thecontrol electrode circuit of the oscillator substantially offset andcancel the effect of the voltage changes that are injected through theanode circuit of the oscillator, when the anode potential source variesin magnitude. In this manner, although the variations in the circuitpotentials are not actually eliminated, their effects are nullified. Inaddition, certain of the circuit potentials are controlled in suchmanner that the cut-ofi potential and the voltage drop in the saturatedanode-cathode discharge path of each tube are made variable, andeachassumes a minimum value thatis near zero during that portion of theoscillation cycle in which changes in its value would be reflected .in

changed oscillation characteristics.

A more complete understanding of the invention and the manner in whichthe foregoing objects are accomplished may be obtained from thefollowing description of two embodiments thereof, when considered inconjunction with the drawing, in which:

Fig. 1 is an explanatory graph of the variations in potential on thecontrol electrode of one electron discharge device of a multivibrator.Reference is made to this graph in the following description;

Fig. 2 is a schematic circuit diagram of a multivibrator oscillator inaccordance with the present invention which is stabilized againstfrequency variations arising from changes: in the magnitude of the anodepotentialsupply; and

Fig. 3 is a schematic circuit-diagram of a multivibrator oscillator inaccordance with the invention in which the oscillation characteristic isstabilized against changes arising both from variations in the anodepotential supply and from variations arising through changes in theelectron discharge devices as they age.

In the following descriptions and in Figs. 2. and 3 of the drawing, likereference characters have been used to denote the same or similarcircuit elements.

A more complete understanding of this invention may be had if it isfirst noted that several conditions contribute .to instability of "theoscillation frequency in the conventional relaxation oscillator of :themultivibrator type. In general, the oscillation frequency is largelydetermined by the time constants of the inter- -electrode couplingcircuits of the oscillator. Tosa lesser, but still considerable degree,the frequency is also determined by the magnitude of the potentialsupply source, the load impedance, and the characteristics of theelectron discharge devices or tubes themselves. Of these last-namedfactors, variations in the first are likely to occur at any time in theoperating cycle, While variations in the last tend to occur primarily asaging of the tube progresses. It is thought that these factors affectthe oscillation frequency of the oscillator through the variations thatthey introduce into the tubes cut-off potential value Eco, the magnitudeof the voltage drop in the anode-cathode path of the tube when it issaturated, and the potential at which the control electrode of thesaturated tube rests in the instant before that tube changes to itscutoff condition. The interrelation of these factors,

associated cathode electrodes.

and the manner in which they are controlled through the practice of thisinvention, may be best understood if the explanatory grid potential waveform diagram of Fig. 1 is first considered, together with themultivibrator oscillator circuit of Fig. 2.

Although in the following discussion the abovementioned factors will bedescribed as affecting only the potential change on control grid ll ofelectron discharge device [0, it will be understood that they exertexactly the same influence on the potential of control-grid electrode [3of discharge device l2. Therefore, variations in these factors areeffective in causing variations in the oscillation frequency through thecontrol that they exert in each'of the electron discharge devices.

Fig. l is a graphical representation of the various circuit potentialdifferences that influence the oscillatory period of the oscillator. Italso forms a' convenient basis for understanding how the stabilizationof the oscillator against the efiect of changes in the anode potentialsupply is accomplished. In this figure, t1 indicates the cut-01finterval of discharge device H! '(see Fig. 2), which'interval it isdesired to maintain constant; E1 indicates the difference in potentialbetween a suitable reference potential, such as ground, and thepotential of the anode when the discharge device or tube [2 issaturated;E2 indicates the difference in potential between this same referencepotential and that of the control electrode of tube I just prior to thetime that this tube is cut oil"; the broken line Eco indicates the valueof the cut-off potential of electron discharge device or tube Ill forthe designed operating conditions; Ep denotes the total potentialdifierence that is available for affecting the momentary potential ofcontrol electrode H and is numerically equal to the negative value ofthe anode or plate potential source or battery I B; KEp denotes thepotential to which the controlgrid electrodes are connected, and whichthey would tend to assume if current conduction were not started in theelectron discharge device when its control-grid electrode acquired apotential in excess of Eco; and (0) indicates the actual ground andcathode potential.

Referring now to Fig. 2, the electron discharge devices I0 and 12, whichmay suitably be pentode type vacuum tubes, together with the anode loadresistors l4, IS; the coupling capacitors 2B, 22; the grid resistors 24,26; and the source of anode potential or battery I8, constitute aconventional multi-vibrator oscillator in which the controlgridelectrodes II and i3 are connected to a point of potential that ispositive with respect to the This positive biasing potential is securedfrom the movable arm 3| of potentiometer 32, which potentiometer forms avoltage dividing network across the potential source or battery l8. Inaccordance with one feature of this invention, the arm of potentiometer32 is so positioned that a fractional part, KEp, of the anode supplypotential Ep is supplied to the control electrodes ii and it throughgrid leak resistors 24, 26. This supplied potential KEp is critical, andis so proportioned that when added to the cut-oii potential Eco acombined potential value Eb (Fig. 1) results. At the time of cut-off,the potential of control electrode I l becomes Ea volts more negativethan its cut-off value Eco. When the grid charging potential KE isproperly chosen, the ratio Eb/Ea is a constant value for varying valuesof E Lil potential on control electrode uated The optimum value ofpotential KEp is secured by properly positioning the movable arm ofpotentiometer 32, and it be recognized by the fact that if KEp is lessthan its optimum value, the frequency of oscillation varies inverselywith an increment of potential change in anode potential source [8; thatis, an increase in the potential of source l8 results in the lowering ofthe oscillation frequency. Conversely if KE is greater than its optimumvalue, the frequency of oscillation varies directly with changes in thepotential of source H8. The actual value of the optimum grid chargingpotential KE will vary as the circuit parameters are changed. However,for each circuit configuration there is one distinct value of biasingpotential KEp for which the ratio Eb/Ea (Fig. 1) remains constant forall reasonably expectable potential changes in the anode potentialsupply Ep.

The manner in which the proper adjustment of the control grid chargingpotential KEp introduces the desired compensatory effect may be bestunderstood if the operation of the circuit (Fig. 2) is examined at atime just priorto and at the time that electron discharge device I0changes from its saturated-current condition to its cut-off condition.At the time just prior to being cut off, discharge device it issaturated; its control grid II is resting at a potential which may beslightly in excess of ground potential. At the instant of cut-off, thepotential on control electrode ll descends Ea volts below its cut-off"potential to a value 5!! that is determined by the charge then existingacross coupling capacitor 22 and the change in po tential at the anodeof the now conducting device l2. This potential Ell is numerically equalto the anode supply potential Ep minus the combined residual potentialsE1 and E2, these latter being the potential difference between the anodeand cathode electrodes of the saturated device !2 and the potential oncontrol electrode I! during the current saturation period of device it),respectively. After being driven negatively, the H increases ascapacitor 22 charges until a potential of Ea) is reached, at which timeconduction is again started in device N). This potential excursion Eamay be evaluated as:

If current conduction were not started in device it] when its controlelectrode acquired the potential Eco and capacitor 22 continued tocharge, the potential on control electrode would become asymptotic tothe biasing potential KEp, as denoted by the dotted line 54. This Wouldrepresent a further voltage excursion:

Therefore, as the potential of control electrode I I rises from its mostnegative point or value 58 to its cut-oif potential Eco at point 52, itfollows the exponential condenser charging curve evalas g10:KE(Ep+KEp-El-E2.) 6 where T represents the time constant of capacitor 22and resistor 24 (Fig. 2). If 6 10 is equated to the cut-off potential--Eco, the cut-off interval t1 of device it may be evaluated as:

cut-off interval n will remain constant notwithstanding variations inthe anode potential supply t =T log llv E5 iftheresultant changes -thatare introduced into potentials E1; E2; and- Ea, are caused to becompensatory; or-if E1,- E2, and Eware caused to have zero values:Inthecircuit configuration shown in Fig. 2,,tl'1'efirst' of'theseconditions, namely;- that the caused variations in potentials Er, E2;and 'Eco are compensatory is-brought about bythc proper proportioningof'the control grid biasing-- potential KEp in the manner which has beendescribed. Inthis (Fig. 2) arrangement; the variations that areintroduced into potentials-E1; E2; and Eco by changes in the anodepotentialsupply IB-may or-maynot be proportional. Itais' not:necessarythat they be proportional; it is-only necessary: that theeffect of thesevariations becom-pensating in such manner that the above mentione'dlogarithmic quantity maintains a" constant value;Thiscompensatingcondition is obtainable ineach case when the -gridbiasing potential KEp issuitably chosen.

In-the--circuitarrangement of Fig. 3, increased frequency stabilityis-obtai-ned by combining. the previously described arrangement withafurther improvement. The variations in potentials E1, Errand-"Eco;caused bychanges-in the-anode potential supply; are made compensator-y"through theproper choice of the grid biasing potential KEp." whileatthesame time the potentials E1 and-Em arecaused tohaveaminimum'valueduring that por-tionofthe operating cycle of eachelectrondischarge'device- I0 or -l2during which they-exert-a controllinginfluence on the oscillation frequency. Sincetheselatter potentialsarecaused to assume=minimum values approaching zero vaiue an-y variationsin the'n rwill' necessarily also besmaii, and the disturbing effect ofsuch variations will; be minimized if not completely eliminated.

Inthercircuit arrangement of Fig. 3, likereferenceucharacters are'usedto-denote circuit elements that are-the sameas or similar to thoseshown=in themultivibrator circuit of Fig. 2. It will be noted that theportion of the circuit comprising electron discharge devices" I'D and I2is substantially the same as-in Fig. 2, with the exceptiorrthatthe-screen grid potentials -for these devices are madevariable-insteadof being fixed quantities as in the formercaser As inthe former case the control-grid electrode biasing potential KEsissecuredat the movable arm 3| of potentiometer 32. Each screen-gridelectrodeis biased from. a variablevoltage-dividing circuitcomprisingres-istor 28 o-r 30 andthe anode-cathode discharge path of anauxiliary electron discharge device-34m 36, each of which maysuitablybea pentode typevacuum tube orother suit able controll'ablyvariable-impedance element. The-screen' electrode: potential forauxiliary: de= vices 34; 36 is obtained through a suitably proportioned.resistor *fromrthe-anode potential source: Hi. The control electrodes,31 of these devices are connected through capacitors 38', 39 tortheanodes of: discharge: devices I01. and I2; Grid-'ileak-.resistors ill;44 are also aconnectedito these control electrode:..circuits and" are;of .such magnitude that time-constant 1 circuits. which: arelongcrelative to r the .:oscillation'iperiodv. of tubes .1 D or 1:2result; from their 'combinationt withrtherre spective capacitors :38 33.

The manner in. whichthe multivibratorztype of oscillator of" Fig; 3operates 'to: achieve increased frequency'stability will besbetter:understood if it is =recalledthat the saturated current conductioncondition of oneoscillator tube coincides with the currentcut-offcondition of the other tube; and

' zero or thereabouts.

vice versa: It should also be recalle'dlthatvatiationsthe-potential"difierence Ei =between-: the anodeand cathode --electrodesin the saturated tube are efiective in-infiuencing the oscillationfrequency during thesame portionof the cycle that variations -=in -=the=eut-off potential value areefiective in the non-conducting or cut-offtuber From thepreviouslydescrib'ed circuit relations it will be recalledthat the cut-off interval" i1 is evaluated! It is, therefore, evident tthat: increased stability in th'ecut-ofiinterval 'trvwill result'if inaddition tolcausing the variations in potentials E1, E2 and Eco to becompensatory (as was described in con nection with Fig, 2)thepotentialsEr andEca are causeto assume minimum val'uesnearzero duringthat portion 7 of j the oscillation cycle in which they exert acontrollinginfluence. Thesescombined stabilizing influences, arerealized in the multivibratorv type? of circuit of Fig. '3 throughthecombination of the compensating adjustment of the controlelectrode.potentials, 'and'the ac tion of the variable impedanceelements, pentodes34, 36; on the screen electrode potentials off'oscillator tubes 10, 12.

Both E1 and Eco are greatly influenced butin oppcsed'directions, bychanges in the magnitude of thescreen electrode potential. That is, anin:- creasein this potential reduces the potential 'dif ference Eibetween the anode and cathode-electrodesof the tube while ,it is initssaturated conditicn, which is desirable; but it also increases thecut-off potential Eco, which is undesirable" Since E1 and Eco existsimultaneously butrin different tubes, it is possible .to reduce each toa minimum at thesameti'menby properly. controlling the screen electrodepotential of the respective tube.- Accordingly, separate screenelectrodebiasing'cir-v cuits including variable impedance devices 34, 36areprovided for theelectron discharge devices-.10 and I2. When electrondischarge device ortube l0 changes from its full conductionto itscut-off condition, a positive voltagetimpul's'e is coupled from itsanode circuit through; coupling capacitor 38to control electrode 33 ofthe variable impedance device or pentode 34. Theltime constantofcapacitor 38 and resistor 44 is large, and this positive voltage impulsepersistsxon electrode:33 for the duration of the cut-off interval iii tolower the impedance of thexdi'schargepath of pentode 34*,and'therebylower the potential of the screen electrodeof device 10..This "lowered screen elec trodezpotential reduces the cut-off" potentialEco to a, minimum value which may bem'adeto be Any changes in the anodepotential supply Ep or in the spacing of the elements of dischargedevice 10; which "change would normally tend to affect the-cut-ofipotenthereby-increase thenimpedancel of the.:a'nod'ecath'odev'diseharge:path: of this auxiliaryrpentode: This increasedimpedance increasesthemotential on: the screena electrode of: devices12;? 5 which.

change decreases the voltage drop E1 in the saturated anode-cathodedischarge path of device I2 to a minimum value that is considerably lessthan it would be if the screen electrode potential were not increased.Because of the minimum value condition, the effect of variations incircuit potentials or structures, which variations would normallyinfluence or change the anode-cathode potential difference, iseliminated or at least minimized.

The action of the auxiliary discharge devices 34, 36 is thus seen to beto increase the screen electrode potential of the saturated oscillatortube and thus lower the voltage drop in its anodecathode discharge path,while at the same time the'screen electrode potential of the cut-off ornon-conducting device is lowered and the cut-off potential Eco of thisdevice is reduced to a mini.- mum value.

In addition to the stabilizing action of the auxiliary pentodes 34, 36,it will also be understood that the selection of the optimum biasingpotential KEp for the control grid electrodes H and [3 of devices Hi andI2 imparts a stabilizing effect against variations caused by changes inthe magnitude of the anode potential supply E8. The correct positioningof the movable arm SI of potentiometer 32 to secure this optimum biasingpotential KEp is performed in the manner previously described inconnection with Fig. 2. The empirical method for determining the correctposition of this potentiometer arm may be performed as described, butthe actual value of KEp will usually be found to be different in thearrangements of Figs. 2 and 3 because of the revised conditions thatgovern the potentials E11 and Eco in the latter arrangement.

In general, the most satisfactory method of stabilizing the describedtype of oscillator will employ both the choice of an optimum value ofbiasing potential KEp and the use of variable screen electrodepotentials on the oscillator tubes. Through the combined use of thesestabilizing factors, an extremely high degree of frequency stability maybe achieved. However, it should be appreciated that this invention isnot limited to only the combination of these two stabilizing factors,since a material but lesser improvement in the stability of theoscillator may be secured through the use of only variable screenelectrode potentials on the oscillator tubes.

Although this invention has been described as being incorporated in aspecialized type of oscillator, it will be evident to those skilled inthe art that it is not limited in its application to the describedarrangements. Therefore, it is to be expected that variations of thisinvention, which do not depart from its spirit and scope, will suggestthemselves to those skilled in the pertinent art.

What is claimed is:

. l. A stabilized multivibrator type oscillator comprising a first and asecond pair of electron discharge devices, each of said devicescomprising anode, cathode, control grid and screen grid electrodes, 2.source of positive potential resistively connected across theanode-cathode electrodes of each of said devices, a capacitive couplingbetween the anode and electrode of each one and the control gridelectrode of each other one of said first pair of devices,grid-leakresistors connected in the control grid cathode circuit of eachof said devices, the screen grid electrode of each of said first pair ofdevices being connected to the anode-electrode of a respective onewhereby the impedance of of said second pair of devices, and acapacitive coupling connecting the anode electrode of each of said firstpair of discharge devices and the control grid electrode of the samerespective one of said second pair of devices.

2. A stabilized multivibrator type oscillator comprising a pair ofelectron discharge devices each including an anode, a cathode, a controlgrid and a screen grid electrode, a source of potential, an individualresistive connection between the positive terminal of said potentialsource and each of said anode and said screen grid electrodes,cross-couplings between the anode and each device and the control gridelectrode of each other device, a grid-leak resistor in the control gridcathode circuit of each device, an individual variable impedanceconnected between the negative terminal of said potential source and thescreen grid electrode of each of said discharge devices, and meansconnected to the anode of each discharge device to control the magnitudeof the impedance connected to the screen electrode of said device ininverse relation to the changes in potential of said anode.

3. A stabilized multivibrator type oscillator comprising a pair ofelectron discharge devices, each of which includes an anode, a cathode,'a control grid and a screen grid electrode, a source of potentialhaving positive and negative terminals, a load impedance connectedbetween said positive terminal and each of said anodes, a grid- .leakresistor connected between each control grid electrode and a point ofpotential positive with respect to said cathodes, each of which isconnected to said negative potential terminal, a pair of voltagedividing circuits each connected be.- twcen said positive and negativeterminals and each comprising a fixed and a variable impedance elementin series connection, each of said screen grid electrodes beingconnected to an intermediate point on a respective one of said dividercircuits and a coupling between each anode electrode and the variableimpedance element connected to its conjugate screen grid electrode saidelement ,is changed as the potential of said anode is changed.

4. A stabilized multivibrator type oscillator in accordance with claim 3in which the impedance of said variable impedance element is increasedas the potential of said anode is decreased and vice versa.

5. A stabilized multivibrator type oscillator comprising a pair ofelectron discharge devices, each of which includes an anode, a cathode,a control grid and a screen grid electrode, a source of potential havinga positive and a negative terminal, a load impedance connected betweensaid positive terminal and each of said anodes, a grid-leak resistorconnected between each control grid electrode and its associated cathodeelectrode, which latter is connected to said negative potentialterminal, a pair of voltage dividing circuits connected between saidpositive and said negative terminals each circuit comprising a fixed andvariable impedance element in series connection, said variable elementincluding the anode-cathode discharge path of an auxiliary electrondischarge device which device includes a control grid electrode, aconnection between each of said screen grid electrodes and anintermediate point on a respective one of said divider circuit, and acapacitive coupling between each of said anode electrodes and thecontrol grid electrode of the auxiliary discharge device that isconnected to its conjugate screen grid electrode whereby the potentialof said auxiliary control grid electrode is increased and the impedanceof the auxiliary devices anode-cathode path is decreased as thepotential of the connected anode of said first-mentioned device isincreased, and vice versa.

6. A stabilized multivibrator type oscillator comprising a pair ofelectron discharge devices each of which includes an anode, a cathodeand a screen grid electrode, a source, of current, said source includingpositive and negative terminals, a load impedance connected between saidpositive terminal and each of said anodes, a connection between each ofsaid cathodes and said negative terminal, a grid-leak resistor connectedbetween each control grid electrode and its associated cathode, a pairof voltage dividing circuits connected in parallel between said positiveand negative terminals, each of said circuits comprising a fixedresistance and the anode-cathode path of i an auxiliary dischargedevice, which device includes a control grid electrode, a connectionbetween each of said screen grid electrodes and an intermediatepotential point on a respective one of said divider circuits, acapacitive connection between the anode-cathode circuit of each of saidpair of discharge devices and the control grid electrode of a respectiveone of said auxiliary discharge devices, whereby the impedance of theanode-cathode path of said auxiliary device is varied in accordance withchanges in the magnitude of the current flowing in the anode-cathodecircuit to which its control electrode is connected.

7. A stabilized multivibrator type oscillator comprising a pair ofelectron discharge devices each including an anode, a cathode, a controlgrid and a screen grid electrode, anode-cathode circuits including asource of anode potential and control grid cathode circuits therefor,crosscouplings between the anode-cathode circuit of each device and thecontrol grid cathode circuit of the other device, an impedanceconnection between said source and each of said screen grid electrodes,and means for controlling the potential on each of the screen gridelectrodes in accordance with the magnitude of the current flowing inits associated anode-cathode path, said means comprising an auxiliarydischarge device respective each screen grid electrode, said auxiliarydevice including an anode, a cathode and a control grid electrode, afirst connection between each of said screen grid electrodes and theanode of the respective auxiliary device, and a second connectionbetween the anode-cathode circuit of each of said first-mentioneddevices and the control grid cathode circuit of a respective one of saidauxiliary devices.

8. A multivibrator type relaxation oscillator comprising a pair ofelectron discharge devices each including an anode, a cathode, a controlgrid and a screen grid electrode, a source of positive potential, meanscomprising a plurality of resistive connections for supplying positivepotential from said source to said anode and said screen gridelectrodes, a direct connection from said cathodes to the negativeterminal of said potential source, means comprising a pair of resistiveconnections for supplying a lesser positive potential from said sourceto each of said control grid electrodes, a pair of auxiliary electrondischarge devices each of said devices comprising an anode, a cathodeand a control grid electrode, a connection between the screen gridelectrode of each of said first-mentioned discharge devices and theanode of a respective one of said auxiliary devices, and a capacitivecoupling between the anode of each of said first-mentioned devices andthe control grid electrode of the respective connected auxiliarydischarge device.

9. A relaxation type of oscillator comprising a pair of electrondischarge devices each including an anode, a cathode, a control grid,and a screen grid electrode, a source of positive potential, a resistiveconnection between each anode electrode and said source, a resistiveconnection between each screen grid electrode and said source, acapacitive coupling between the anode of each discharge device and thecontrol grid electrode of the conjugate device, conductive connectionsbetween each control grid electrode and said source of positivepotential, and means for con- REFERENCES CITED The following referencesare of record in the file of this patent:

FOREIGN PATENTS Number Country Date 217,614 Switzerland Feb. 16, 1942563,794 Great Britain Aug. 30, 1944 OTHER REFERENCES Time Bases byPuckle-line 19 on page 26 through line 6 on page 30December 19, 1947.

